In 1930, it was found by Langmuire and Blodgett that a fatty acid having about 16 to 22 carbon atoms could form a monomolecular film on the surface of water and the film could be built up on a substrate but the application of the built-up film has recently begun to be investigated. However, a Langmuir-Blodgett film (hereinafter, is referred to as LB film) of a straight chain saturated fatty acid is insufficient in heat resistance and mechanical strength for practical application and such a LB film cannot be used for practical purpose as it is.
For overcoming these problems, a built-up film of a film formed from a saturated fatty acid such as .omega.-tricocenoic acid, .omega.-heptadecenoic acid, .alpha.octadecylacrylic acid, etc., or an unsaturated fatty acid ester such as vinyl stearate, octadecyl acrylate, etc., has been investigated but such a film is yet insufficient in heat resistance, etc.
On the other hand, a polyimide film is known as a heat resisting film but in the case of forming the film thereof by spin coating, etc., which is known to be a coating method for forming the thinnest film of such a polymer, the thickness of the film formed is at best not thinner than 1000 .ANG. and is usually thicker than about 1 .mu.m. In other words, it is very difficult to form a heat resisting thin film of thinner than 1000 .ANG. having no pin holes by using such a polymer.